Active pixel sensors are well known in the art. A basic active pixel sensor is described in U.S. Pat. No. 5,471,515. An active pixel sensor typically includes a plurality of pixel circuitry units. Each unit, or “pixel” includes a photoreceptor, a buffer, and a select transistor. Some means of resetting the pixel is also provided.
It is desirable to make each pixel as small as possible to enable more pixels to fit on a single chip. For example, a desirable size in current technology is 5×5 μm2. It is also desirable to keep the supply voltages low as possible. For example, it would be useful if an active pixel sensor could operate from a single 1.5 volt battery. This provides limitations on the way in which such a device can operate.